Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some subcarriers to be assigned to different users, rather than to a single user. Today, OFDM and OFDMA technology are used in both wireline transmission systems, such as asymmetric digital subscriber line (ADSL), and wireless transmission systems, such as IEEE-802.11a/g (i.e., WiFi), IEEE-802.16 (e.g., WiMAX), digital audio broadcast (DAB), and digital video broadcast (DVB). This technology is also used for wireless digital audio and video broadcasting.
OFDM networks support the transmission of both broadcast traffic, intended for multiple subscriber stations (i.e., user devices), and unicast traffic, intended for a single subscriber station. Conventional OFDM networks time-multiplex broadcast and unicast traffic in the downlink (i.e., forward channels) by transmitting broadcast and unicast traffic in different downlink transmission time intervals. Accordingly, broadcast traffic may be transmitted in a first transmission time interval (TTI), while unicast traffic is transmitted in at least one TTI other than the first TTI. In general, the duration of each TTI is fixed. The number of OFDM symbols within a TTI may be different for broadcast traffic and unicast traffic. In general, a smaller number of OFDM symbols are carried in a broadcast TTI in order to allow for a longer cyclic prefix (CP).
By way of example, an OFDM network may transmit a 5 millisecond frame in the downlink. Each downlink frame contains eight transmission time intervals, where each TTI is 0.625 milliseconds in duration. Every fourth TTI is reserved for broadcast traffic. Each unicast TTI contains K OFDM symbols and each broadcast TTI contains less than K OFDM symbols.
The signal-to-interference and noise ratio (SINR) for unicast traffic may be written as:
                                          SINR            unicast                    =                      P                          fP              +                              N                0                                                    ,                            [                  Eqn          .                                          ⁢          1                ]            where the value P represents the received power at the subscriber station from the same cell and the value f represents the ratio between other cell and same cell signals. In an interference limited situation, which is the case for most cellular deployments, fP>>N0. Therefore, SINR may be written as:
                              SINR          unicast                =                              P                          fP              +                              N                0                                              =                                    P              fP                        =                                          1                f                            .                                                          [                  Eqn          .                                          ⁢          2                ]            It should be noted that increasing the power, P, does not help to improve unicast SINR.
In the case of broadcast traffic using OFDM, the signals received by a subscriber station from multiple synchronized base stations are orthogonal as long as the relative delays of the received signals are within the OFDM symbol cyclic prefix length. Therefore, there is no interference when the same broadcast content is transmitted system-wide, apart from the background noise. The average SINR in an OFDM-based broadcast is given as:
                                          SINR            broadcast                    =                      KP                          N              0                                      ,                            {                  Eqn          .                                          ⁢          3                }            where the value P is the received power from one base station at the subscriber station and the value K is the number of base stations from which broadcast content is received, assuming equal power is received from K base stations. It should be noted that increasing transmit power results in a linear increase of broadcast SINR.
However, conventional OFDM networks that time-multiplex broadcast and unicast traffic suffer wasted power during unicast traffic transmissions. The reason for this wasted power is that transmitting at a higher power does not help to improve SINR for unicast traffic during unicast traffic transmission periods, due to increased interference from neighboring cells. As a result, the same performance may be achieved by transmitting unicast traffic at reduced power. However, the additional available power cannot be used for broadcast traffic, because broadcast traffic transmissions occur in different time slots (i.e., TTIs) than unicast traffic transmissions. Since either broadcast traffic or unicast traffic, but not both, may be transmitted during a given TTI, it is not possible to allocate the available downlink power adaptively between unicast and broadcast traffic. This results in system inefficiency
Therefore, there is a need for improved OFDM (or OFDMA) transmission systems that make better use of the available downlink transmit power.